1. Field of the Invention
This invention relates to a document transport device for use in an image forming apparatus such as an electrostatic photography type copying machine and a facsimile machine.
2. Description of the Background Art
FIG. 1 is a partially cross sectional view of an image forming apparatus incorporated with a document transport device of prior art. As shown in FIG. 1, the conventional document transport device has a drive roller 32 above a document positioning member 35 that is arranged at an end portion of a contact glass 31 (leftmost end in FIG. 1) with respect to a document transport direction shown by the arrow X. The positioning member 35 is adapted for temporarily pausing feed of a document and positioning the document relative to the contact glass 31 at a certain position for image reading.
A driven roller (not shown) is arranged at the opposite end (rightward direction in FIG. 1) of the contact glass 31 in the document transport direction X. An endless document transport belt 33 is wound around the pair of drive roller 32 and the unillustrated driven roller.
The document transport device further comprises several presser rollers 34 between the drive and driven rollers to press the belt 33 against the upper surface of the contact glass 31 at several positions. Note that only one presser roller 34 closest to the drive roller 32 is shown in FIG. 1.
When the belt 33 is driven between the drive roller 32 and the driven roller, a certain portion (hereinafter referred to as "document guide portion" and shown by the broken line in FIG. 1) 33a of the belt 33 between the drive roller 32 and the presser roller 34 arranged closest thereto leaves away from the contact glass 31, thereby defining a document guide clearance C between the document guide portion 33a and the upper surface of the contact glass 31.
With this arrangement, a document is guided in the document guide clearance C via a document inlet port 36 defined between the positioning member 35 and the drive roller 32.
If a document has a curled portion at an end thereof, such document is guided in the document guide clearance C without correcting the curl. Accordingly, a shade or a black image corresponding to the curled portion of the document is formed on a copy sheet after an image formation because the curled portion does not properly make contact with the contact glass 31.
To prevent such drawback, generally, the document transport device has a sub roller 37 which is movable up and down on the contact glass 31 to be away from the contact glass 31 and in pressing contact therewith. The sub roller 37 is arranged near the positioning member 35 and is connected to a bias member (such as a spring) which applies a bias force to move the sub roller 37 downward toward the contact glass 31 to set the document guide portion 33a around the sub roller 37 in pressing contact with the contact glass 31. With this arrangement, the document guide portion 33a of the belt 33 between the drive roller 32 and the presser roller 34 is pressed against the contact glass 31 with an attempt to prevent a poor image formation due to a curl of an original document.
The above arrangement has suffered from the following drawback because it was aimed to reduce the document guide clearance C as much as possible by the downward movement of the sub roller 37 to prevent a poor image formation due to a curl of an original document. Specifically, when pressing the belt guide portion 33a against the contact glass 31, a frictional force caused between the belt guide portion 33a and the contact glass 31 increases. As a result, a load exerted to a belt drive motor (not shown) to drive the drive roller 32 to forward the belt 33 overcoming the frictional force becomes large, resulting in the necessity of providing a belt drive motor with a large torque force to reliably transport the document over the contact glass 31. The provision of belt drive motor having such large torque force raises the production cost of the document transport device, and cost rise of the image forming apparatus as a whole will result.